As sea raider said, fierds was and still is reliable overall, with rough and tough infantry and tolerable cavalry in the nobility. The fact that they are one of only two factions to have real elites in their peasant troop tree is also a big deal, especially for the AI lords who ALWAYS get 30-40+ elites, every single damn time, within a couple of weeks of getting flattened, without having to spend the month you would have to spend to raise an elite troop. Huscarls and 'Zerkers are both POWERFUL siege units and fantastic at bogging down and chopping up cavalry.
The other top tier option is always going to be the baccus empire, since they are the other only source of Peasant Elites, as well as another faction with large amounts of powerful infantry, and crossbows to boot! Empire gladiators are essentially 'Zerkers, and Legionnares aren't quite elite but they're certainly a damn sight stronger than other level 35 peasant troops.
Think of this as nothing more than a Native Mod for those seeking a challenge. In native, the Nords and Rhodoks are always going to be top tier. It's the same in this: the Fierdsvain and Empire are just in-depth reskins of the Nords and Rhodoks with a completely different story, but from a gameplay perspective they serve the same function more or less, and remain on top of the food chain. If you prefer hack and slash action, massed infantry, and a little chaos, go with the Fierdsvain. If you prefer orderly advances of lines of crossbowmen and shields, with some surprisingly decent cavalry, go for Empire.